The present invention relates to a tire for motorcycles, and more particularly to a tire for motorcycles which has an excellent durability in high speed running of the motorcycles.
With recent prevalence of pavement, it has been intended to raise the speed of vehicles and, therefore, motorcycle tires are also required to possess characteristics capable of accommodating the high speed running.
Generally, for the carcass of motorcycle tire, widely employed is a cross-ply structure, that is, such a structure that ply cords of the carcass intersect each other at an angle of about 15.degree. to 60.degree. with respect to the circumferential direction of the tire. The reason is that the tire for motorcycle is basically different from that for four-wheel vehicles such as passenger car, particularly, in motional functions at the time of turning, and is required to have characteristics to maintain stable running while resisting centrifugal force acting upon the motorcycle body with a force (camber thrust) generated when the motorcycle is tilted during turning at a large angle to a plane perpendicular to the road surface (when a large degree of camber angle is given), which horizontally acts along the direction in which the camber angle is given. Therefore, the motorcycle tire is strengthened in transverse stiffness by employing a carcass of the cross-ply structure as described above so as to maintain the camber thrust. The radial structure inferior in transverse stiffness has been scarcely adopted for the motorcycle tires from the above-mentioned point of view. However, the tire of the cross-ply structure has the drawbacks that transverse vibration (weave phenomenon) of the motorcycle body resulting from cornering power and transverse stiffness of the tire occurs during high speed running and that the abrasion resistance is low. As countermeasures to such problems, the cord angle of carcass to the circumferential direction of the tire is lowered or the number of carcass plies is increased for increasing the tire stiffness, but the drawbacks attributable to the structure cannot completely be removed as far as the cross-ply structure is employed.
Radial tires are superior in steering stability in high speed running and abrasion resistant, though they have the drawback as mentioned above. In order to eliminate the drawback while utilizing these excellent characteristics, it is proposed to arrange bead apexes made of a rubber having a high hardness or reinforcing layers of fiber cords extending from a bead portion to a side wall portion, as disclosed in Japanese Unexamined Patent Publication (Tokkyo Kokai) No. 22704/1983. According to such a structure, the transverse stiffness of the tire is increased and the camber thrust is greatly improved. However, the flexing and deforming zone of the tire shifts from the side wall portion to the shoulder portion of the tire, and it issues another problem that a stress is caused conjointly with the above zone being located at the breaker end which is high in stiffness and consequentially cracks are generated at the bottoms of shoulder grooves positioned at the both end portions of the tread.
It is an object of the present invention to solve the problems arising from adopting the radial structure.
A further object of the present invention is to provide a radial tire for motorcycles having an improved durability.
These and other objects of the present invention will become apparent from the following description and the accompanying drawing.